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- // Copyright Louis Dionne 2013-2017
- // Distributed under the Boost Software License, Version 1.0.
- // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
- #include <boost/hana.hpp>
- #include <string>
- #include <vector>
- namespace hana = boost::hana;
- struct yes { std::string toString() const { return "yes"; } };
- struct no { };
- //! [optionalToString.sfinae]
- template <typename T>
- std::string optionalToString(T const& obj) {
- auto maybe_toString = hana::sfinae([](auto&& x) -> decltype(x.toString()) {
- return x.toString();
- });
- return maybe_toString(obj).value_or("toString not defined");
- }
- //! [optionalToString.sfinae]
- int main() {
- BOOST_HANA_RUNTIME_CHECK(optionalToString(yes{}) == "yes");
- BOOST_HANA_RUNTIME_CHECK(optionalToString(no{}) == "toString not defined");
- {
- //! [maybe_add]
- auto maybe_add = hana::sfinae([](auto x, auto y) -> decltype(x + y) {
- return x + y;
- });
- maybe_add(1, 2); // hana::just(3)
- std::vector<int> v;
- maybe_add(v, "foobar"); // hana::nothing
- //! [maybe_add]
- }
- }
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